This invention relates to a system for continuously monitoring the contents of the individual constituents of a sheet material and, more particularly, to a system for monitoring the contents of the constituents of a paper web.
Various additives such as titanium oxide (TiO.sub.2), calcium carbonate (CaCO.sub.), and clay or kaolin (Al.sub.2 O.sub.3 .multidot.2SiO.sub.2 .multidot.2H.sub.2 O) are often used to improve such paper qualities as brightness and opacity. Considerations of cost and quality control make it desirable that the concentrations of these additives, collectively referred to as "ash", be continuously monitored. Prior attempts to develop on-line monitoring have proven to be deficient in one or more respects. Thus, devices using preferential absorption of gamma rays or backscattering of beta particles to measure total ash content are subject to inaccuracies resulting from variations in the relative concentrations of the ash constituents. A recent device using preferential X-ray absorption attempts to avoid this problem by choosing the X-ray energy such that the absorption coefficients of clay and TiO.sub.2 are equal. This approach, too, has drawbacks. First, the X-ray energy selected is just below the K-absorption edge of titanium, so that only a slight variation in the X-ray energy will produce a large and misleading variation in the measured absorption coefficient. Secondly, since it is not possible to obtain equal absorption coefficients for TiO.sub.2, CaCO.sub.3, and clay at the same X-ray energy level, the presence of CaCO.sub.3 cannot be measured by this system and would in fact introduce error. Finally, this system is only capable of measuring the total ash content and cannot separately measure the clay content and the TiO.sub.2 content. Individual monitoring and control is important, however, since TiO.sub.2 is considerably more expensive than clay.